Components manufactured by many manufacturing processes, such as casting, extrusion, injection molding, vapor deposition, hot pressing, rolling, etc., rarely have the surface quality desired in the finished product, regardless of the method used to define surface roughness. For example, fabricated ceramic and powder metallurgy parts typically have an rms surface roughness in the range of submillimeters to tens of micrometers (microns) upon completion of the initial forming steps. Considerable time and energy is typically expended in performing rough and then fine grinding on the surfaces of such parts to produce the near-final shapes, and also to ensure a degree of surface roughness that is either sufficient for final use or sufficient as a starting point for ultra-fine grinding and polishing.
Because the final grinding and polishing steps are typically very expensive, a manufacturer is required to accurately determine when an optimum ground surface quality has been achieved, so that the expensive grinding operation can be terminated. However, with many known devices for measuring surface roughness, the part must be removed from the grinding and/or polishing jig and transported to a separate, often off-line test bed or chamber with a measuring device, in order to perform the surface characterization measurements. Accordingly, it has been particularly time consuming, and concomitantly expensive, to perform these surface measurements because the part must be frequently removed and evaluated several times before terminating the final grinding and polishing steps.
Typical apparatus for measuring surface roughness are stylus-type instruments, such as the device referred to as the Talysurf, which include a stylus or probe which is placed in contact with the surface to be measured and is moved along a linear dimension of the surface. The probe moves up and down with the contour of the surface, and thus provides an indication of the roughness of the surface. Because the stylus or probe must contact the surface in order to provide measurement of the surface roughness, however, the probe can scratch the surface, thus requiring further polishing and/or fine grinding which is an added expensive processing step. The test piece whose surface is to be measured also typically has to be removed from the grinding or polishing jig and then mounted on the measurement instrument to take the measurements. These types of instruments also typically require periodic calibration in order to account for the wear of the probe and to provide consistently accurate measurements. The measurements are also indicative of the roughness of only the specific points contacted by the stylus or probe in a linear dimension, thus often requiring a large number of measurements to accurately determine the surface roughness of an area of a surface.
Another method for measuring surface roughness is referred to as the Lichtschnitt method (from the German for "light-cut"). The Lichtschnitt method of surface topography measurement is well known in classical optical microscopy. An illuminated knife-edge is focused sharply on the target surface, at some oblique angle of incidence such as 45.degree.. When viewed from above, the shadow edge will appear to be a rough (or tortuous) line separating light from shadow; the rougher the surface, the rougher this line. From geometry, measurements of the displacements of the shadow edge from a true straight line then give the heights of the peaks and valleys of the rough surface. This method, generally used in conjunction with microscope viewing of the target surface, has been used particularly to measure step-heights in the 0.5-50 micron step range. This method has generally not been used for the evaluation of rough surfaces, however, because of the tediousness of measurements and because of poor statistics resulting from the relatively short length of optical scan of the surface (as limited by the field of view of the microscope).